1. Field of the Invention
The present invention relates to a method of positionally controlling a recording/reproducing head over tracks of a recording disk and a disk apparatus which is controlled by such a method, and more particularly to a method of positionally controlling a recording/reproducing head along circular patterns on a recording disk around the axis of rotation thereof and a disk apparatus which is controlled by such a method.
2. Description of the Related Art
As well known in the art, there are ever-increasing demands for magnetic disk drives with greater storage capacity. In order to increase the storage capacity of magnetic disk drives, it is necessary to reduce the pitch of tracks, i.e., track-to-track distances, of magnetic disks. Reducing the track pitch requires tracks having servo information, i.e., servo tracks, to be written accurately in position on magnetic disks. It has been desired in the art to manufacture inexpensive magnetic disk drives with accurate servo tracks on magnetic disks.
FIG. 13 of the accompanying drawings illustrates how a servo track deflects to an eccentric position with respect to a circular pattern around the axis of rotation of a magnetic disk. FIG. 14 of the accompanying drawings illustrates the manner in which a track deviates from a circular pattern on a magnetic disk. FIG. 15 of the accompanying drawings shows a conventional magnetic disk drive for controlling the position of a magnetic head.
In order to write servo tracks accurately in position on magnetic disks, it has heretofore been necessary to provide individual magnetic disk drives with a function to write servo tracks accurately in position on the magnetic disks. However, magnetic disk drives with such a function are relatively expensive to manufacture.
To avoid such a drawback, it has been proposed to write servo tracks on magnetic disks with a servo track writer, and thereafter install those magnetic disks in individual magnetic disk drives.
Specifically, servo tracks are written on magnetic disks by a servo track writer having a high positioning accuracy. Then, the magnetic disks with the servo tracks written thereon are installed respectively in individual magnetic disk drives. This process eliminates the need for providing individual magnetic disk drives with a function to write servo tracks accurately in position on the magnetic disks. Consequently, it is possible to manufacture inexpensively magnetic disk drives having servo tracks with a small track pitch.
When a magnetic disk with a servo track written thereon is installed in a magnetic disk drive, as shown in FIG. 13, a circular servo track 14-1 carrying servo information on a magnetic disk 14 may be positioned out of alignment with a circular pattern 14-2 around the axis of rotation of the magnetic disk 14. Specifically, since the magnetic disk 14 with the circular servo track 14-1 written thereon is subsequently installed in the magnetic disk drive, the circular servo track 14-1 is often not aligned with the circular pattern 14-2 even in the presence of a small installation error.
The magnetic disk 14 is liable to deflect to an eccentric position because of such an installation error of the magnetic disk 14. As shown in FIG. 13, a magnetic head 13 supported on an arm 17 moves radially over the magnetic disk 14. The magnetic head 13 positioned along the circular pattern 14-2 on the magnetic disk 14 is controlled by the servo information carried by the servo track 14-1, and positioned along the servo track 14-1.
As shown in FIG. 14, the servo track 14-1 suffers a track deviation from the circular pattern 14-2 on the magnetic disk 14, the track deviation containing an eccentric deflection. Since the track deviation contains an eccentric deflection, the magnetic head 13 is subjected to a large positional error, and hence is likely to vibrate during operation of the magnetic disk drive.
Efforts have been made to apply an eccentric control process to eliminate the above shortcoming. According to such an eccentric control process, as shown in FIG. 15, a magnetic disk drive 12 has a magnetic disk 14 and a spindle motor 15 for rotating the magnetic disk 14. A magnetic head 13 is mounted on the distal end of an arm 17 which is movable by a rotary actuator (VCM) 10 for positioning the magnetic head 13 radially over the magnetic disk 14.
A head position detector 20 detects the position of the magnetic head 13 from a signal read from the magnetic disk 14 by the magnetic head 13. A control processor 25 effects a control process (e.g., PID operation) on a positional error to calculate a control current. The control current is amplified by an amplifier 23, and supplied to energize the VCM 10.
While the arm 17 is being fixed, the magnetic head 13 reads servo information from the magnetic disk 14 and measures a positional error. A positional error, i.e., the track deviation shown in FIG. 14, for one revolution of the magnetic disk 14 is read, and stored in a memory 22. In a normal control mode, the track deviation stored in the memory 22 is added to the positional error from the magnetic head 13 for controlling the VCM 10.
In this manner, an eccentric component (track deviation) is measured to control the magnetic head 13 according to a feedforward control process. Therefore, the magnetic head 13 is positioned along a circular pattern on the magnetic disk 14, thus minimizing the positional error.
As described above, it has been customary to measure a track deviation and effect a feedforward control process to eliminate the measured track deviation for thereby positioning the magnetic head 13 along the circular pattern on the magnetic disk 14.
FIGS. 16 and 17 of the accompanying drawings illustrate problems with the conventional eccentric control process.
If the track pitch is reduced, the magnitude of the core width of the magnetic head affects a demodulated output signal. Since there is a certain limit to efforts to reduce the core width of the magnetic head, the detecting area of the magnetic head exhibits sensitivity variations.
Because of such sensitivity variations, a demodulation displacement is not linear with respect to a head displacement, as shown in FIG. 16. Specifically, when the magnetic head is positioned at a track boundary, the servo demodulation displacement is discontinuous. This is because the detecting area of the magnetic head exhibits sensitivity variations if the track pitch is reduced.
In the case where read and write heads separate from each other, such as of an MR head assembly, are employed, the magnetic head is offset in position in order to compensate for the difference between core positions of the read and write heads. With the magnetic head being thus offset, the magnetic head tends to be easily positioned at a track boundary. Discontinuous regions of the servo demodulation displacement exhibit a high-frequency positional error. When a feedforward control process is effected to eliminate a track deviation, an eccentric component is compensated for, but a high-frequency component is emphasized, as shown in FIG. 17. Therefore, the magnetic head vibrates across the center of the track, as shown in FIG. 17. Consequently, the magnetic head tends to vibrate excessively, and its positioning accuracy is lowered.